Optical electrowetting devices are devices capable of modifying an incident beam to achieve a specific optical function. They include variable focus liquid lenses, optical diaphragms, optical zooms, ophthalmic devices and are increasingly proposed in a number of applications and apparatuses, such as for example cameras, cell phones, telemeters, endoscopes, dental video cameras and the like.
An optical lens driven by electrowetting and of variable focal length is described for example in European Patent EP-B1-1,166,157, the content of which is incorporated herein by reference. FIG. 10 of the present application corresponds to FIG. 12 of that patent. A cell is defined by a fluid chamber comprising a lower plate 10 and an upper plate (not shown), and a perpendicular (normal to), or substantially perpendicular (normal to), axis 0. The lower plate, which is non-planar, comprises a conical or cylindrical depression or recess 6, which contains a non-conductive or insulating fluid 2. The remainder of the cell is filled with an electrically displaceable conductive fluid 8 along the axis 0.
The fluids are non-miscible, in contact over a meniscus (A, B), and have a different refractive index and substantially the same density. The cell comprises an electrical insulating substrate 4, arranged on at least an area of the lower plate 10, on which both fluids are in contact.
The thickness of the insulating substrate is preferably comprised between about 0.1 μm and about 100 μm. Generally, small thickness is to be used for optical electrowetting devices working at low voltage, whereas thick insulating substrate is used for high voltage applications.
The conductive fluid generally is a salt containing-aqueous fluid. The insulating fluid is typically an oil, an alkane or a mixture of alkanes, possibly halogenated.
The optical quality of an optical electrowetting device may vary in the conditions of use, depending on various parameters.
Importantly, the optical liquid lenses driven by electrowetting may present a focal hysteresis, meaning that their optical powers differ depending on voltage ramp direction, increasing or decreasing. In other words, the focal length of the device at a given voltage value may be different depending on whether the tension is increasing or decreasing and the interface between the conductive and the non-conductive fluids is moving towards or inwards with respect to the axis of the cell. It has been found that this phenomenon is related to contact angle hysteresis. It has finally been found that a degradation of the optical quality is associated with such a hysteresis.